Magnetic sensor device

ABSTRACT

Provided is a magnetic sensor device capable of performing signal processing at high speed with high accuracy. The magnetic sensor device includes: a plurality of Hall elements; a plurality of differential amplifiers to which the plurality of Hall elements are connected, respectively; a detection voltage setting circuit for outputting a reference voltage; and a comparator including: a plurality of differential input pairs connected to the plurality of differential amplifiers, respectively; and a differential input pair connected to the detection voltage setting circuit.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Japanese PatentApplication No. 2013-032117 filed on Feb. 21, 2013, the entire contentof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic sensor device which convertsa magnetic field intensity into an electric signal, and, for example,relates to a magnetic sensor device used as a sensor for detecting anopen/close state of a folder-type cellular phone, a notebook computer,or the like or a sensor for detecting a rotational position of a motor.

2. Description of the Related Art

As a sensor for detecting an open/close state in a folder-type cellularphone, a notebook computer, or the like or a sensor for detecting arotational position of a motor, a magnetic sensor device is used.

A magnetic sensor device outputs a voltage in proportion to a magneticfield intensity or a magnetic flux density by a magnetoelectricconverting element (for example, Hall element), amplifies the outputvoltage by an amplifier, makes a determination using a comparator, andoutputs a binary signal of an H signal or an L signal. An output voltageof a magnetoelectric converting element is minute, and thus, an offsetvoltage of the magnetoelectric converting element (element offsetvoltage), an offset voltage of the amplifier or the comparator (inputoffset voltage), or noise generated in the magnetic sensor devicebecomes a problem. The element offset voltage is generated mainly by astress applied to the magnetoelectric converting element from a packageor the like. The input offset voltage is generated mainly by variationsin the characteristics of an element which forms an input circuit of theamplifier. The noise is generated mainly by flicker noise of a singletransistor which forms a circuit or thermal noise of a single transistoror a resistance element.

A magnetic sensor device which reduces the effect of an offset voltageof the above-mentioned magnetoelectric converting element or amplifierhas been invented (see, for example, Japanese Patent ApplicationLaid-open No. 2010-281801). A related-art magnetic sensor deviceillustrated in FIG. 4 includes a Hall element 51 which is amagnetoelectric converting element, a changeover switch circuit 52, adifferential amplifier 53, a comparator 54, a detection voltage settingcircuit 55, a first capacitor C51 and a second capacitor C52, and afirst switch S51 and a second switch S52.

The differential amplifier 53 has an instrumentation amplifierconfiguration as illustrated in FIG. 5, and includes differentialamplifiers 61 and 62 and resistors R61, R62, and R63. Each of thedifferential amplifiers 61 and 62 operates as a noninverting amplifier.A first input terminal of the differential amplifier 53 is connected toa noninverting input terminal E61 of the differential amplifier 61, asecond input terminal of the differential amplifier 53 is connected to anoninverting input terminal E62 of the differential amplifier 62, afirst output terminal of the differential amplifier 53 is connected toan output terminal E63 of the differential amplifier 61, and a secondoutput terminal of the differential amplifier 53 is connected to anoutput terminal E64 of the differential amplifier 62. The differentialamplifier 53 having such instrumentation amplifier configuration enablesinhibiting the effect of in-phase noise in differential input. In thiscase, it is assumed that the amplification factors of the differentialamplifiers 61 and 62 are set to be equal to each other.

FIG. 6 shows a timing chart of an operation of the related-art magneticsensor device. A cycle T of a detection operation is divided into afirst detection state T1 in which a power supply voltage is input to afirst terminal pair A-C of the Hall element 51 and a detection voltageis output from a second terminal pair B-D by the operation of theabove-mentioned changeover switch circuit 52, and a second detectionstate T2 in which the power supply voltage is input to the secondterminal pair B-D and a detection voltage is output from the firstterminal pair A-C by the operation of the above-mentioned changeoverswitch circuit 52. Further, the cycle T is divided into a first samplephase F1, a second sample phase F2, and a comparison phase F3 by openingand closing the respective switches. In the comparison phase F3, offsetcomponents are removed.

However, in the related-art magnetic sensor device, time-divisionoperation in which a plurality of signal processing periods such as asample phase and a comparison phase is necessary to be provided for thepurpose of cancelling out offset components, which is inappropriate forhigh speed signal processing. Further, the time-division operationrequires connection of a switch circuit and a capacitor element, whichcomplicates the circuit configuration.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magnetic sensordevice which utilizes a comparator including a plurality of Hallelements and a plurality of differential input pairs to cancel outoffset components of the Hall elements, thereby realizing a highlyaccurate magnetic field intensity detection, and at the same time,performing high speed signal processing.

In order to solve such related-art problems, a magnetic sensor deviceaccording to one embodiment of the present invention has the followingconfiguration.

The magnetic sensor device includes: a plurality of Hall elements; aplurality of variable resistors connected to the plurality of Hallelements, respectively; a plurality of differential amplifiers connectedto the plurality of Hall elements, respectively; and a comparatorincluding a plurality of differential input pairs connected to theplurality of differential amplifiers, respectively.

According to the magnetic sensor device of one embodiment of the presentinvention, the detection voltage level of the magnetic field intensitycan be arbitrarily set by a small scale circuit, and thus, the Hallelement offset can be cancelled out, and at the same time, the signalprocessing can be performed at high speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a magnetic sensor deviceaccording to an embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a Hall element and a variableresistor according to the embodiment.

FIG. 3 is an exemplary circuit diagram of a comparator used in theembodiment.

FIG. 4 is a circuit diagram of a related-art magnetic sensor device.

FIG. 5 is an exemplary circuit diagram of a differential amplifier ofthe related-art magnetic sensor device.

FIG. 6 is a timing chart of the related-art magnetic sensor device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention is described in detail in thefollowing with reference to the attached drawings. A magnetic sensordevice according to the present invention is widely used as a sensor fordetecting the state of a magnetic field intensity, for example, a sensorfor detecting an open/close state in a folder-type cellular phone, anotebook computer, or the like or a sensor for detecting a rotationalposition of a motor. In the following embodiment, a magnetic sensordevice using Hall elements is described, but the magnetic sensor deviceaccording to the present invention may use, instead of a Hall elementwhich outputs a voltage in accordance with a magnetic field intensity, aconverting element which similarly outputs a voltage in accordance witha physical quantity such as acceleration or a pressure.

FIG. 1 is a circuit diagram illustrating the magnetic sensor deviceaccording to this embodiment. The magnetic sensor device according tothis embodiment includes Hall elements 1 a and 1 b for outputting asignal voltage in accordance with a magnetic field intensity,differential amplifiers 2 a and 2 b for amplifying the signal voltage, acomparator 3 including two differential input pairs, and variableresistors Ra and Rb connected to the Hall elements.

The Hall elements 1 a and 1 b are arranged on a semiconductor substrateso as to be close to each other, and so that a straight line connectinga first terminal pair A-C of the Hall element 1 a and a straight lineconnecting a first terminal pair E-G of the Hall element 1 b areparallel to each other. As a result, a straight line connecting a secondterminal pair B-D of the Hall element 1 a and a straight line connectinga second terminal pair F-H of the Hall element 1 b are also parallel toeach other. The differential amplifiers 2 a and 2 b have aninstrumentation amplifier configuration illustrated in FIG. 5 which isreferred to in the description of the related art. The comparator 3 hasa circuit configuration illustrated in FIG. 3, and the detaileddescription thereof is given below. A voltage VO of an output terminalOUT of the comparator 3 is expressed by Expression (1):VO=A1×(V6−V5)+A2×(V8−V7)  (1)

In Expression (1), A1 and A2 are amplification factors of twodifferential amplifiers, respectively, which form the comparator 3. Thevariable resistors Ra and Rb are connected between a ground and aterminal of the Hall element 1 a connected to the differential amplifier2 a, and between the ground and a terminal of the Hall element 1 bconnected to the differential amplifier 2 b, respectively.

Next, an operation of the magnetic sensor device according to thisembodiment is described. Transmission of signal components is describedunder the assumption that a differential output voltage at outputterminal pairs of the Hall elements 1 a and 1 b with respect to amagnetic field component is denoted by Vh, the element offset voltage isdenoted by Voh, the in-phase voltage is denoted by Vcm (≈VDD/2), theamplification factor of the differential amplifiers 2 a and 2 b isdenoted by G, and detection voltage components which are determined bythe variable resistors Ra and Rb and are output to the output terminalpairs of the Hall elements 1 a and 1 b are denoted by Vrefa and Vrefb,respectively. The direction of a current flow in the Hall element 1 aand the direction of a current flow in the Hall element 1 b form anangle of 90 degrees, and thus, the offset component in the outputterminal pair of the Hall element 1 a and the offset component at theoutput terminal pair of the Hall element 1 b are in reverse phase. Fromthe above description, signal voltages at the respective points arecalculated as follows:V1=Vcm−Vh/2+Voh/2  (2)V2=Vcm+Vh/2−Vrefa−Voh/2  (3)V3=Vcm−Vh/2−Voh/2  (4)V4=Vcm+Vh/2−Vrefb+Voh/2  (5)V5=Vcm−G(Vh/2−Voh/2)  (6)V6=Vcm+G(Vh/2−Vrefa−Voh/2)  (7)V7=Vcm−G(Vh/2+Voh/2)  (8)V8=Vcm+G(Vh/2−Vrefb+Voh/2)  (9)

Substituting the above-mentioned values V5 to V8 into Expression (1)yields Expression (10) which expresses the voltage VO. The differentialamplifiers in the comparator 3 are generally identical with one another,and thus, A1=A2=A holds.VO=AG(2Vh−Vrefa−Vrefb)  (10)

It can be understood that, in this way, the offset components of theHall elements 1 a and 1 b are cancelled out, and a comparison can bemade between the amplified signal component of the magnetic fieldintensity and the arbitrarily set reference voltage components. In theabove-mentioned calculation, the assumption is made that the variableresistor Ra is connected between the terminal B of the Hall element 1 aand the ground and the variable resistor Rb is connected between theterminal G of the Hall element 1 b and the ground. However, a similarresult is obtained when the one end of the variable resistors areconnected not to the terminal B but to the terminal D and not to theterminal G but to the terminal E, respectively. In this case, thepolarity of the detected magnetic field intensity is reversed. Further,a similar result is also obtained when the other ends of the variableresistors are connected not to the ground terminal GND but to a powersupply voltage terminal VDD.

In this embodiment, the voltage VO is an output voltage of a comparator(the value of A is very large), and thus, has an H signal (VDDpotential) or an L signal (GND potential) in accordance with the valueof Vh. Further, it can be understood that the series of signalprocessing does not require time-division signal processing unlike therelated-art case, which enables high speed signal processing. A switchcircuit and a capacitor element which are required for the time-divisionsignal processing are unnecessary, which contributes to chip sizereduction, that is, cost reduction.

Here, Vrefa or Vrefb which is the above-mentioned detection voltagecomponent is described. As illustrated in FIG. 2, the Hall elements 1 aand 1 b are represented as equivalent bridge circuits includingresistors R1 to R4 and R5 to R8, respectively. One end of the variableresistor Ra is connected to the terminal B of the Hall element 1 a andthe other end of the variable resistor Ra is connected to the ground.Similarly, one end of the variable resistor Rb is connected to theterminal G of the Hall element 1 b and the other end of the variableresistor Rb is connected to the ground. Next, the operation isdescribed. When the potential of the terminal B and the potential of theterminal D are equal to each other in the Hall element 1 a, that is,when the potentials of input terminals of a first differential inputpair of the comparator provided on the subsequent stage are equal toeach other, the relationship expressed by Expression (11) holds:Ra=R1*R2*R3/(R2*R4−R1*R3)  (11)

In Expression (11), it is assumed that R1=R2=R3=R4=R holds under a statein which there is no magnetic field, and the amount of change inresistance values of the resistors when a certain magnetic field isapplied thereto is denoted by ΔR. Then, in the state expressed byExpression (11), R1=R−ΔR, R2=R+ΔR, R3=R−ΔR, and R4=R+ΔR can be thoughtto hold. Substituting these into Expression (11) yields Expression (12):Ra=R ²*(1−ΔR/R−(ΔR/R)²+(ΔR/R)³)/(4*ΔR)  (12)

ΔR is sufficiently small compared with R, and thus, the quadratic andcubic terms with respect to (ΔR/R) are negligible. Then, Expression (13)holds:ΔR/R≈1/(1+4*Ra/R)  (13)

Therefore, according to this embodiment, irrespective of the powersupply voltage and variations in manufacture, the detection voltage canbe determined only by a resistance ratio Ra/R. As a result, a highlyaccurate detection voltage level setting can be realized. Similarly,with regard to the Hall element 1 b, the detection voltage can bedetermined by the variable resistor Rb. From the above description, thedetection voltage components Vrefa and Vrefb are derived as follows:Vrefa≈1/(1+4*Ra/R)*VDD  (14), andVrefb≈1/(1+4*Rb/R)*VDD  (15)

Further, the comparator 3 is described. The comparator 3 has a circuitconfiguration illustrated in FIG. 3, and includes a constant currentcircuit I1, NMOS transistors M43, M44A, M44B, M45A, M46A, M45B, andM46B, and PMOS transistors M41 and M42. The comparator has the followingconnections. One end of the constant current circuit I1 is connected toa power supply voltage terminal VDD, and the other end is connected to adrain and a gate of the NMOS transistor M43. VBN denotes this node. VBNis connected to gates of NMOS transistors M44A and M44B. Sources of theNMOS transistors M43, M44A, and M44B are connected to a ground terminalVSS. Sources of the NMOS transistors M45A and M46A are connected to adrain of the NMOS transistor M44A, sources of the NMOS transistors M45Band M46B are connected to a drain of the NMOS transistor M44B. Drains ofthe NMOS transistors M45A and M45B are connected to a drain of the PMOStransistor M41. VA denotes this node. Drains of the NMOS transistorsM46A and M46B are connected to a drain of the PMOS transistor M42. Thisnode is connected to the output terminal OUT of the comparator 3. Gatesof the PMOS transistors M41 and M42 are connected to the node VA, andsources of the PMOS transistors M41 and M42 are connected to the powersupply voltage terminal VDD. Gates of the NMOS transistors M45A and M46Aare respectively connected to a second input terminal V6 and a firstinput terminal V5 of a first differential input pair. Gates of the NMOStransistors M45B and M46B are respectively connected to a second inputterminal V8 and a first input terminal V7 of a second differential inputpair.

Next, an operation of the comparator 3 is described. The constantcurrent circuit I1 generates a constant current and supplies theconstant current to the NMOS transistor M43. The NMOS transistors M43,M44A, and M44B form a current mirror circuit. A current based on acurrent which flows between the drain and the source of the NMOStransistor M43 flows between the drains and the sources of the NMOStransistors M44A and M44B, respectively. The five transistors of theNMOS transistors M44A, M45A, and M46A and the PMOS transistors M41 andM42 form a differential amplifier. The differential amplifierconfiguration operates so that the difference in gate voltage betweenthe NMOS transistors M45A and M46A, that is, the difference in voltagebetween the second input terminal V6 and the first input terminal V5 ofthe first differential input pair, is amplified to be output to theoutput terminal OUT. A1 denotes this amplification factor. Operations ofthe current mirror circuit configuration and the differential amplifierconfiguration are described in detail in literature with regard to aCMOS analog circuit and the like, and thus, detailed description thereofis omitted here. Further, the five transistors of the NMOS transistorsM44B, M45B, and M46B and the PMOS transistors M41 and M42 also form adifferential amplifier, and operate so that the difference in gatevoltage between the NMOS transistors M45B and M46B, that is, thedifference in voltage between the second input terminal V8 and the firstinput terminal V7 of the second differential input pair, is amplified tobe output to the output terminal OUT. A2 denotes this amplificationfactor. In addition, the drains of the NMOS transistors M45A and M45Bare connected via the node VA to the drain of the PMOS transistor M41,and the drains of the NMOS transistors M46A and M46B are connected viathe output terminal OUT to the drain of the PMOS transistor M42. Withthe above-mentioned configuration, the signal voltages which have beeninput to and amplified by the respective differential amplifiers areadded at the node VA and the output terminal OUT. The operation isexpressed by above-mentioned Expression (1).

Note that, with respect to the embodiment, by respectively increasingthe number of the Hall elements and the number of the variable resistorsto, for example, four, increasing the number of the differential inputpairs in the comparator to four (increasing the number of the inputterminals to eight) accordingly, and thereby effectively inhibiting theeffect of variations in the offsets of the Hall elements, the accuracyof detecting the magnetic field intensity can be further enhanced. Inthis way, the present invention can accommodate a configuration whichutilizes a comparator including a plurality of Hall elements and aplurality of differential input pairs.

Further, by replacing the comparator with a differential amplifier, themagnetic sensor device described in the embodiment of the presentinvention can have a configuration which outputs an analog signal.

What is claimed is:
 1. A magnetic sensor device for outputting a signalin accordance with an intensity of a magnetic field applied to a Hallelement, the magnetic sensor device comprising: a plurality of Hallelements; a plurality of variable resistors connected to the pluralityof Hall elements, respectively; a plurality of differential amplifiers,each having a first input terminal and a second input terminalrespectively connected to an output terminal of one of the plurality ofHall elements, respectively; and a comparator including a plurality ofdifferential input pairs connected to the plurality of differentialamplifiers, respectively.
 2. A magnetic sensor device according to claim1, wherein the plurality of differential amplifiers comprise: a firstdifferential amplifier further comprise: a first output terminal and asecond output terminal for outputting amplified signal voltages; and asecond differential amplifier comprising: a first input terminal and asecond input terminal respectively connected to an output terminal pairof a second Hall element; and a first output terminal and a secondoutput terminal for outputting amplified signal voltages, wherein theplurality of variable resistors comprise: a first variable resistorconnected between anyone of the output terminal pair of the first Hallelement and anyone of a power supply terminal and a ground terminal; anda second variable resistor connected between anyone of the outputterminal pair of the second Hall element and anyone of the power supplyterminal and the ground terminal, and wherein the comparator comprises:a first differential input pair comprising a first input terminal and asecond input terminal respectively connected to the first outputterminal and the second output terminal of the first differentialamplifier; and a second differential input pair comprising a first inputterminal and a second input terminal respectively connected to the firstoutput terminal and the second output terminal of the seconddifferential amplifier.